Event Triggered Execution: AppInit DLLs

Adversaries may establish persistence and/or elevate privileges by executing malicious content triggered by AppInit DLLs loaded into processes. Dynamic-link libraries (DLLs) that are specified in the AppInit_DLLs value in the Registry keys HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Windows or HKEY_LOCAL_MACHINE\Software\Wow6432Node\Microsoft\Windows NT\CurrentVersion\Windows are loaded by user32.dll into every process that loads user32.dll. In practice this is nearly every program, since user32.dll is a very common library. [1]

Similar to Process Injection, these values can be abused to obtain elevated privileges by causing a malicious DLL to be loaded and run in the context of separate processes on the computer. [2] Malicious AppInit DLLs may also provide persistence by continuously being triggered by API activity.

The AppInit DLL functionality is disabled in Windows 8 and later versions when secure boot is enabled. [3]

ID: T1546.010
Sub-technique of:  T1546
Platforms: Windows
System Requirements: Secure boot disabled on systems running Windows 8 and later
Permissions Required: Administrator
Effective Permissions: Administrator, SYSTEM
Data Sources: Command: Command Execution, Module: Module Load, Process: Process Creation, Windows Registry: Windows Registry Key Modification
Version: 1.0
Created: 24 January 2020
Last Modified: 10 November 2020

Procedure Examples

ID Name Description
G0087 APT39

APT39 has used malware to set LoadAppInit_DLLs in the Registry key SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows in order to establish persistence.[4]

S0107 Cherry Picker

Some variants of Cherry Picker use AppInit_DLLs to achieve persistence by creating the following Registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows "AppInit_DLLs"="pserver32.dll"[5]

S0458 Ramsay

Ramsay can insert itself into the address space of other applications using the AppInit DLL Registry key.[6]

S0098 T9000

If a victim meets certain criteria, T9000 uses the AppInit_DLL functionality to achieve persistence by ensuring that every user mode process that is spawned will load its malicious DLL, ResN32.dll. It does this by creating the following Registry keys: HKLM\Software\Microsoft\Windows NT\CurrentVersion\Windows\AppInit_DLLs – %APPDATA%\Intel\ResN32.dll and HKLM\Software\Microsoft\Windows NT\CurrentVersion\Windows\LoadAppInit_DLLs – 0x1.[7]

Mitigations

ID Mitigation Description
M1038 Execution Prevention

Adversaries can install new AppInit DLLs binaries to execute this technique. Identify and block potentially malicious software executed through AppInit DLLs functionality by using application control [8] tools, like Windows Defender Application Control[9], AppLocker, [10] [11] or Software Restriction Policies [12] where appropriate. [13]

M1051 Update Software

Upgrade to Windows 8 or later and enable secure boot.

Detection

Monitor DLL loads by processes that load user32.dll and look for DLLs that are not recognized or not normally loaded into a process. Monitor the AppInit_DLLs Registry values for modifications that do not correlate with known software, patch cycles, etc. Monitor and analyze application programming interface (API) calls that are indicative of Registry edits such as RegCreateKeyEx and RegSetValueEx. [1]

Tools such as Sysinternals Autoruns may also be used to detect system changes that could be attempts at persistence, including listing current AppInit DLLs. [14]

Look for abnormal process behavior that may be due to a process loading a malicious DLL. Data and events should not be viewed in isolation, but as part of a chain of behavior that could lead to other activities, such as making network connections for Command and Control, learning details about the environment through Discovery, and conducting Lateral Movement.

References